Document snapshots.l.o push set-up

[Данило Шеган]

When we upload artefacts from android-build.l.o jenkins master to snapshots.linaro.org we use a slightly involved process to ensure secure behaviour and make it work with existing jenkins plugins like ssh plugin:
 - push files to a temporary chrooted location on snapshots.linaro.org
 - call a script over ssh after pushing which moves fully pushed files to the final location for them

This set-up needs to be documented so anyone from the team can jump in and see how it works and modify it.